A facile synthesis of a-amino-DOTA as a versatile molecular imaging probe Byunghee Yoo and Mark D. Pagel * Department of Biomedical Engineering, Case Western Reserve University, 10900 Euclid Avenue Cleveland, OH 44106-7207, USA Received 5 January 2006; revised 3 August 2006; accepted 9 August 2006 Available online 30 August 2006 Abstract—An amino group has been introduced into one ligand of DOTA that can couple to peptidyl carboxylates by coupling a-brominated glycine to DO3A- t Bu (1,4,7,10-tetraazacyclododecane-1,4,7-tris(acetic acid, tert-butylester)). a-Amino-DOTA was coupled to the carboxylate backbone terminus of a peptide to demonstrate the utility for derivatization. Ó 2006 Elsevier Ltd. All rights reserved. Macrocyclic metal chelates using DOTA are often administered to in vivo patients and animal models to create or enhance contrast in biomedical molecular imaging studies. Examples include paramagnetic Gd 3+ - DOTA for magnetic resonance imaging (MRI), 1 radioactive 111 In-DOTA for SPECT imaging, 2 and radioactive 64 Cu-DOTA for PET imaging. 3 More recently, metal-DOTA chelates have been conjugated to peptides to affect the pharmacokinetics of the me- tal-DOTA imaging agent within in vivo systems, which can be used to gain additional information about biological processes at the molecular level. A variety of peptidyl ligands 4 have been employed for these molecu- lar imaging studies, including peptides that bind to spe- cific cell surface receptors, 5 peptides that penetrate cell membranes, 6 peptides that nonspecifically interact with the extracellular matrix, 7 and very large peptide homo- polymers that drastically alter renal clearance rates. 8 To synthesize these peptidyl imaging agents, the carb- oxylates of DOTA have been conjugated to the amines of peptides, including the N-terminus, the side chain of lysine, and unnatural amino acid derivatives such as p-NH 2 -phenylalanine. 9 Other DOTA derivatives have been devised for conjugation to peptide amino groups, such as succinimide DOTA derivatives 10 and isothiocy- anate DOTA derivatives. 11 However, coupling DOTA only to peptide amines can limit synthesis methodolo- gies. 12 Also, modification of a peptide N-terminus or side chain amino group can compromise the utility of the peptide for molecular imaging applications, because peptide–biomolecule interactions critically depend on the (native) peptide amino acid sequence. To address these limitations, a facile methodology is re- quired for conjugating DOTA to peptide carboxylates, especially the C-terminus of a peptide. We have devel- oped new a-amino-DOTA derivatives to conjugate directly to the C-terminus of a peptide and used two different glycine templates (BOC and CBZ protected) to accommodate future applications via orthogonal pro- tection strategies. BOC protection is more advantageous in solution phase synthesis, and CBZ protection has advantages in orthogonal synthesis and more complex structures. We have also chelated lanthanide ions with these DOTA derivatives. Finally, we have confirmed that a-amino-DOTA-Gd 3+ has good T 1 MR relaxivity to demonstrate that these DOTA derivatives may serve as molecular imaging agents. Two different synthetic pathways for the a-bromination of glycine templates are shown in Scheme 1. The com- pound 2 13 was obtained by following previously reported methods by use of N-bromosuccinimide and filtered UV radiation (254 nm). The bromination showed high efficiency in purity as determined by NMR. 14 The compounds 5–7 15 were synthesized according to a previously reported synthetic pathway 16 and acquired with overall yield of 70% as determined by weight. The compound 9 17 was synthesized from 2 and 8 and purified with a silica column. To continue further 0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2006.08.019 Keywords: DOTA; T 1 relaxivity; MRI; Molecular imaging. * Corresponding author. Tel.: +1 216 368 8519; fax: +1 216 368 4969; e-mail: mpagel@case.edu Tetrahedron Letters 47 (2006) 7327–7330